a) Field of the Invention
This invention relates to ion-selective membranes reinforced with woven-fabric-shaped backings capable of permeating electrolytes therethrough, a process for their production, use of the ion-selective membranes, and apparatuses provided with the ion-selective membranes.
b) Description of the Related Art
Cation-selective or anion-selective membranes and charge mosaic membranes are known to date. A cation-selective or anion-selective membrane, which may hereinafter also be called a xe2x80x9csole ion-selective membranexe2x80x9d, has cationic or anionic polymer components. In a charge mosaic membrane, on the other hand, a cationic polymer component and an anionic polymer component extend in substantially electrically (ionically) continuous forms in the direction of the thickness of the membrane. A sole ion-selective membrane has function that counter ions can be either adsorbed or permeated but non-counter ions are repelled, while a charge mosaic membrane has function that electrolytes of low molecular weights can be permeated but non-electrolytes are not permeated or are permeated at very low permeation rates.
For the production of the above-described sole ion-selective membranes or charge mosaic membranes, the following processes have been proposed: (1) concerning sole ion-selective membranes, (i) fine powder of an ion exchange resin and a matrix-forming thermoplastic resin, such as polyethylene or polyvinyl chloride, are kneaded into a homogeneous mass, and the homogeneous mass is then formed under heat, (ii) triethylamine, chlorosulfonic acid or the like is reacted directly to a film of polyethylene, polyvinyl chloride or the like such that ionic: groups are introduced onto the film, and (iii) (meth)acrylic acid is graft-polymerized on a polyethylene, polypropylene or like film or a membrane obtained by graft polymerization of styrene or vinylpyridine is sulfonated or quaternized, such that ionic groups are introduced; and (2) for charge mosaic membranes, block copolymers are used.
The process (1)(i), which makes use of an ion exchange resin the particle size of which is coarse, is easy to practice, but has difficulty in making the concentration of immobilizable ions higher because the resin particles has a small specific surface area. The process (1)(ii), in which ionic groups are introduced onto a membrane by treatment through a chemical reaction, is cumbersome to practice. On the other hand, the process (2) making use of a block copolymer is by itself very difficult to practice. In contrast, the process in which a cationic component and/or an anionic component is used in the form of fine polymer particles has a merit in that the production of a sole ion-selective membrane or charge mosaic membrane is very easy.
In the above-described process making use of fine polymer particles, use of the fine polymer particles in the form of microspheres makes it possible to rather easily; produce an ion-selective membrane owing to the inherent close packability and isotropy of the microspheres. Nonetheless, this process is accompanied by a problem for the production of a membrane of large area, because the components of the membrane are ionic polymers and the membrane so formed tends to shrink or lose strength upon drying.
As a process which reduces the above problem, a process making use of a pliable polymer component as a matrix component for a charge mosaic membrane is proposed in JP Kokai No. 10-87855. This process has facilitated the production of a charge mosaic membrane of large area. However, this process requires long time for the formation of the membrane and moreover, has difficulty in producing a thin membrane having a uniform thickness. As processes capable of lessening this problem, it is proposed in Japanese Patent Applications Nos. 10-232732 and 10-237708 to form a membrane from a composition in which ionic polymer components are dispersed in a solution of a resin as a matrix component, such as a polysulfone resin, a polyarylate resin or a polyurethane resin, in an organic solvent; and to use a backing such as a woven fabric or a nonwoven fabric in combination. These processes have made it possible to shorten the membrane-forming time and to obtain an ion-selective membrane having a uniform and thin thickness and permitting easy handling.
However, formation of an ion-selective membrane, which has been produced by using a backing such as a woven fabric or non-woven fabric in combination, into a shape suited for incorporation in an actual apparatusxe2x80x94such as a planar membrane, a spiral membrane, a corrugated membrane, a cylindrical membrane or hollow capillariesxe2x80x94involves problems in that the permeation performance for electrolytes is reduced because of pin-holes or crazing formed in the membrane due to variations or nonuniformity of openings in the backing or because of nonuniformity in thickness of membranes formed around the openings in the backing. There is hence an outstanding desire for the resolution of this problem.
An object of the present invention is therefore to provide an ion-selective membrane, which makes use of ionic polymer component or components and a matrix component, is resistant to the development of pinholes or crazing upon formation or handling of the membrane, and is uniform in thickness around openings of a backing, and has sufficient strength. Another object of the present invention is to provide a process for the production of the ion-selective membrane.
To achieve the above-described objects, the present invention provides, in one aspect thereof, an ion-selective membrane formed integrally of an ion-selective membrane forming component, which comprises a cationic polymer component and/or an anionic polymer component and a matrix component, and a woven-fabric-shaped backing, wherein each ionic polymer component is a particulate polymer having an average particle size in a range of from 0.0 to 10 xcexcm, and said woven-fabric-shaped backing has a meshed structure; in another aspect thereof, a process for the production of the ion-selective membrane; in a further aspect thereof, use of the ion-selective membrane; and in a still further aspect thereof, an apparatus provided with the ion-selective membrane.
The ion-selective membrane according to the present invention is resistant to the development of pin-holes or crazing upon formation or handling, is uniform in thickness around openings of a backing, and retains sufficient mechanical strength.
Therefore, the ion-selective membrane according to the present invention can be used for the selective permeation of an electrolyte, for example, for the transport, separation, concentration, adsorption or the like of an electrolyte.